1. Sanders-Brown Center on Aging, University of Kentucky, Lexington, KY 40536-0305, USA
2. Graduate Center for Gerontology, University of Kentucky, Lexington, KY 40536-0305, USA
3. Department of Anatomy and Neurobiology, University of Kentucky, Lexington, KY 40536-0305, USA
4. Spinal Cord and Brain Injury Research Center, University of Kentucky, Lexington, KY 40536-0305, USA